1. Field of the Invention
The present invention relates to a method for measuring an amount of deformation of an object, and more particularly, to a method for measuring, in a non-contact manner, the deformation of an industrial product caused by being pressed.
2. Description of the Related Art
A speckle interference method is an optical measuring method utilizing a spot-shaped bright and dark pattern (speckle interference pattern) generated when a rough-surfaced object is irradiated with a laser beam. Since the speckle interference pattern is an inherent pattern corresponding to a surface shape of an object to be measured which has been irradiated with a laser beam, the amount of deformation of the observed surface can be estimated from the change thereof.
First, two laser beams are irradiated on the object to be measured, and respective scattering lights generated on the observed surface of the measured object are caused to interfere with each other, and the bright and dark pattern (spot-shaped pattern corresponding to unevenness of the observed surface) is observed, thereby the speckle interference pattern can be obtained. In the speckle interference method, a highly accurate deformation measurement using a wavelength of light as a reference becomes possible.
FIG. 9 illustrates an example of an optical system used in the speckle interference method. A laser beam 1a emitted from a laser beam source 1 is divided into two laser beams by a beam splitter 2 which uses a half mirror or the like. Then, the respective laser beams are reflected by mirrors 3 and 4, and enlarged by the lenses 5 and 5a, and the lenses 6 and 6a, and the two laser beams are irradiated onto an object 7 to be measured.
The scattering lights from the object to be measured generate optical interference with each other, pass through a camera lens 8a, and an image is formed on an imaging surface of a camera 8. A speckle interference pattern acquired by the camera is input into a computer 9.
The speckle interference pattern is imaged again after a predetermined time length, and it is possible to measure what degree of deformation has been generated on the object 7 to be measured, by taking a difference from an original speckle interference pattern.
When the square of difference is calculated with respect to two speckle interference patterns before and after deformation of the object 7 to be measured, a fringe-shaped image (hereinafter, called a speckle interference fringe image) according to the amount of deformation is obtained. Then, the amount of deformation can be estimated from the number of fringes of the speckle interference fringe image. The relationship between the number of fringes and the amount of deformation has a relationship that as the amount of deformation becomes larger, the more fringes are generated.
The amount of deformation Δd per one fringe depends on a wavelength of laser, and an incident angle 10 into the object to be measured 7, and is expressed as equation (1) below, when incident angles of two laser beams are equal to each other,
                              Δ          ⁢                                          ⁢          d                =                              λ                                          2                .                            ⁢                                                          ⁢              sin              ⁢                                                          ⁢              θ                                ⁡                      [            nm            ]                                              (        1        )            
where λ is wavelength of laser beam [nm], and θ is an incident angle [°].
However, as the amount of deformation becomes larger and the number of fringes increases, spatial frequencies of fringes which can be captured by the camera 8 have an upper limit, and thus the fringes are blurred to become unidentifiable. For this reason, the large deformation as the fringes are blurred cannot be measured.
Japanese Patent Application Laid-Open No. 6-94434 discusses a method for measuring a large deformation by temporally dividing and acquiring deformation of the object to be measured, and integrating the amounts of deformation. To describe in detail, when the speckle interference patterns are continuously acquired at the predetermined time intervals, difference between two continuous images is calculated from the acquired speckle interference patterns, and the amount of deformation is determined from the number of fringes. The final amount of deformation is determined by repetitively determining the amounts of deformation between the two speckle interference patterns, and summing them.
As described above, the technique discussed in Japanese Patent Application Laid-Open No. 6-94434 is to estimate the amount of deformation based on the number of fringes. As a minimum unit of the amount of deformation that is actually readable, about half of an amount of deformation of fringe (sub-micron order) is a limit, and as a reliable significant digit of measurement results (hereinafter, referred to as an accuracy), an order of micron becomes a limit.